Folding knives are commonly used by sportsmen, craftsmen and other individuals who desire a lightweight, handheld cutting instrument which can be conveniently stored in a pocket or attached to a belt or other article of clothing with a clip. The folding feature of the knife substantially reduces the overall length and cumbersome nature of the knife as compared to fixed-blade knives. Additionally, during the storage of a folding knife, typically at least a portion of the knife blade is received in a cavity within the knife handle. This prevents the cutting edge of the knife blade from being exposed.
To use the knife blade, the blade is rotated at the point of attachment to the handle and extended outwardly, wherein the knife blade is positioned coextensive with the knife handle. In the extended position, the knife blade is generally "locked" into position by means of a spring-leaf locking arm positioned near the cavity of the knife handle which engages a heel end of the knife blade. Another common type of folding knife locking mechanism utilizes a "back lock" which has a biased arm near the rear of the handle which engages the heel portion of the knife blade when the knife blade is extended. Another type of primary locking mechanism used for folding knives is disclosed in U.S. Pat. No. 5,615,484 to Pittman, which discloses a cam mechanism centered at the point of rotation on the heel end of the blade. The cam mechanism is biased against a locking arm to prevent either the inadvertent opening or closing of a folding knife blade. Based on the design, however, the manufacturing of these types of primary locking mechanisms can be expansive and due to the constant frictional wear, susceptible to failure after extensive use.
Although there are numerous other folding knife primary locking mechanisms, all of them have the same problem of potentially failing after extensive or improper use. Any failure to the locking mechanism can create a danger to the user of the knife since the knife blade can close inadvertently and potentially injure the user. Thus, a significant need exists for a secondary safety locking mechanism which substantially prevents the closing of a knife blade of a folding knife if a failure exists with the primary locking mechanism.